The invention relates to a power supply polarity reversal protection circuit and, more particularly, to a device for protecting a load circuit from a power supply connected with reversed polarity.
Most modern electronic circuits are intolerant of being connected to a power supply with reversed polarity. Reversing the polarity will typically render a circuit inoperative, and may also permanently damage or destroy the more sensitive components in the circuit. The most common method of protecting a circuit from a reversed supply voltage is to place a diode between a power supply terminal and the circuit. The diode will allow current to flow from the power supply to the circuit when the diode is forward biased from a correctly polarized supply, but will not allow current to flow when the power supply is connected in the reverse direction. This method has the disadvantage that there is always a power loss from the current flow through the diode. In addition, there is typically a forward voltage drop of about one volt across the diode, thus reducing the voltage level available to the circuit.
Other methods for protecting a circuit from a reversed power supply involve complex circuitry and specialized integrated circuits. U.S. Pat. No. 4,260,910 to Colman discloses an integrated circuit with a built-in power supply protection. This circuit is not off-the-shelf and includes a transistor, two resistors, and a diode. This circuit provides limited reverse polarity protection through an integrated resistor which may heat up if a power supply is connected with reversed polarity.
U.S. Pat. No. 4,426,658 to Gontowski discloses an integrated circuit with protection against a reversed power supply. The disclosed circuit includes several transistors, diodes, resistors, and a fusible link.
While these methods may be suitable for the particular purpose which they address, or for general use, they would not be suitable for the purpose of the present invention because of their complexity and/or inherent power loss.